1. Field of the Invention
This invention relates to a bicycle front derailleur which is used in cooperation with a front gear comprising a plurality of chain sprockets arranged side by side and having different numbers of teeth, and which is designed to shift a drive chain selectively from one sprocket to another for speed change. More particularly, the invention relates to an improved bicycle front derailleur of the type that is designed for use with a particular type of front gear having three or more chain sprockets, wherein the improvements are made to provide a good performance in shifting the chain from the smallest sprocket to a larger sprocket as well as from an intermediate sprocket to the largest sprocket.
2. Description of Related Art
The conventional bicycle front derailleur generally has a basic arrangement as illustrated in FIGS. 5 and 6. Thus, a description will be first made hereinafter to explain the basic arrangement of the conventional front derailleur with reference to FIGS. 5 and 6.
The conventional bicycle front derailleur has a base member 3 (FIG. 6) fixed to a lower portion of a seat pipe 1 at a position adjacent to a hanger lug by means of a clamp band 2. A movable element 5 is mounted to the base member 3 via a control mechanism such as a parallelogram-shaped pantograph link mechanism 4. The pantograph link mechanism 4 comprises outer and inner links 6, 7 pivoted, by means of pins, at their upper ends to the base member 3 and at their lower ends to the movable element 5. The pins, which connect each upper end of the links 6, 7 to the base member 3, and each lower end thereof to the movable element 5, provide four apexes of the parallelogram, so that a pivotal movement of the links 6, 7 relative to the base member 3 provides dislocation of the movable element 5 with its orientation kept constant. The pantograph mechanism 4 is normally urged in one direction for deformation by a torsion spring 8 which is fitted on the pin connecting the inner link 7 to the movable element 5 and which is engaged at its one end with a certain member on the base member and at its other end with the movable element 5. The pantograph mechanism 4 is also deformable by pivoting the links 6, 7 against the urging force of the spring 8 via a known control cable W.
The movable element 5 is provided with inner and outer guide plates 9, 10 whose side surfaces are substantially parallel to those of sprockets S and whose arcuate lower edges 9a, 10a respectively extend substantially circumferentially about the sprockets S. The guide plates 9, 10 are arranged to flank a part of a drive chain C that is at a point before engagement with a selected one of the front gear sprockets S. Thus, when the movable element 5 moves inwardly, the inner face of the outer guide plate 10 pushes on the aforesaid part of the chain C inwardly to disengage it from e.g. the largest sprocket s3. On the other hand, when the movable element 5 moves outwardly, the inner face of the inner guide plate 9 pushes on the aforesaid part of the chain C e.g. on the smallest sprocket s1 to forcibly shift it into initial engagement with a larger sprocket s2 or s3.
Immediately after the forcible disengagement from the largest sprocket s3 by the outer guide plate 10, the chain C is shifted down onto a smaller sprocket s1 or s2 for full engagement therewith, by a tension force derived from a rear derailleur (not shown). On the other hand, the chain C forced into the initial engagement with e.g. the largest sprocket s3 by the inner guide plate 9 is brought into full engagement therewith as the sprocket s3 rotates.
The conventional front gear of the type that is now popular, typically illustrated in FIG. 5 as having three sprockets, is characterized in that the innermost smallest sprocket s1 among the three is considerably reduced in diameter than the outermost largest sprocket s3 in order to provide an improved climbing ability of a bicycle which permits an easier cycling in running up a slope.
Thus, in a particular type of the conventional bicycle front derailleur for use with the above described multiple front gears, it is customary that a vertical distance between the arcuate lower marginal edge 9a of the inner guide plate 9 and the similar edge 10a of the outer guide plate 10 is much greater than that in an ordinary type of conventional front derailleur so that the lower edge 9a which, at a certain point thereof, will push on the chain C can be disposed as close as possible to the proximity of the circumference of the smallest sprocket s1, as seen in FIG. 5, in order to perform good shifting of the chain C from the smallest sprocket s1 onto the neighboring intermediate sprocket s2. This particular arrangement is advantageous in that, since the lower edge 9a is able to push on the chain C outwardly at a point relatively close to the circumference of the smallest sprocket s1, even a small displacement of the movable element 5 or the inner guide plate 9 permits initial engagement of the chain C with the intermediate sprocket s2 resulting in an immediate response to the gear change operation.
However, with the above arrangement in which the arcuate lower edge 9a is lowered for maintaining its concentric relation with circles defining the circumferences of the sprockets, the following disadvantage is brought about when the chain C is shifted from the intermediate sprocket s2 onto the largest sprocket s3, although the above discussed improvement in shifting the chain C from the smallest sprocket s1 onto the intermediate sprocket s2 is attained.
Because the lower edge 9a of the inner guide plate 9 is disposed lower than that in the ordinary front derailleur, as seen in FIG. 5, the inner face of the inner plate 9 extends downwardly and overlaps the inner side surface of the largest sprocket s3 to a larger extent. As a result, even if the inner guide plate 9 is moved outwardly in an attempt to push on the chain C to move it from the intermediate sprocket s2 toward the largest sprocket s3 for engagement therewith, the chain C is likely to be trapped between the inner guide plate 9 and the largest sprocket s3, as shown in phantom lines in FIG. 6.
Thus, in the prior art front derailleur to be used in cooperation with the triple sprocket type front gear as illustrated in FIGS. 5 and 6, the downwardly extended lower edge 9a of the inner guide plate 9 on one hand results in improved shiftability of the chain from the smallest sprocket s1 to the intermediate sprocket s2, but on the other hand leads to deterioration in shiftability of the chain from the intermediate sprocket s2 onto the largest sprocket s3. As elucidated hereinbefore, this results from the fact that the downwardly extended lower edge 9a of the arcuate inner guide plate 9 pushes the chain C at a point reasonably close to the smallest sprocket s1 when the chain C is shifted from the smallest sprocket s1 onto the intermediate sprocket s2, whereas in shifting of the chain C from the intermediate sprocket s2 onto the largest sprocket s3, the extended lower edge 9a pushes the chain C at a point excessively close to the intermediate sprocket s2.